Subsections

• 2.1.1 Properties of SiO$_2$
• 2.1.2 Structure of SiO${_2}$

2.1 The Material Silicon Dioxide

SiO$_2$ is one of the most important and attractive materials in semiconductor fabrication, especially for MOS technology. In contrast to other materials which suffer from one or more problems, SiO$_2$ offers a lot of desired characteristics and advantages [25,26]:

• SiO$_2$ layers are easily grown thermally on silicon or deposited on many substrates.
• They are resistant to most of the chemicals used in silicon processing and yet can be easily patterned and selectively etched with specific chemicals or dry etched with plasmas.
• They block the diffusion of dopants and many other unwanted impurities.
• The interface that forms between silicon and SiO$_2$ has very few mechanical or electrical defects and is stable over time.
• SiO$_2$ has a high-temperature stability (up to 1600$^{\circ }$C) indispensable for process and device integration.
• SiO$_2$ is an excellent insulator with a high dielectric strength and wide band gap.

2.1.1 Properties of SiO $\boldsymbol{_2}$

Table 2.1: Important properties of SiO${_2}$.

Density (thermal, dry/wet)	2.27/2.18 g/cm$^{-3}$
Thermal expansion coefficient	5.6 $\cdot$ 10$^{-7}$ 1/K
Young's modulus	6.6 $\cdot$ 10$^{10}$ N/m$^2$
Poisson's ratio	0.17
Thermal conductivity	3.2 $\cdot$ 10$^{-3}$ W/(cm$\cdot$K)
Relative dielectric constant	3.7 - 3.9
Dielectric strength	10$^{7}$ V/cm
Energy bandgap	8.9 eV
DC resistivity	$\approx$ 10$^{17}$ $\Omega\cdot$cm

In Table 2.1 some important properties of SiO$_2$ are listed [27]. The density of thermally grown dry oxide is a little bit higher than of wet oxide, which leads to a better oxide quality. The thermal expansion coefficient is a measure of stress or strain, which the oxide exerts on other materials in contact with it, practicularly during high-temperature cycles. The Young's modulus and Poisson's ratio describe the mechanical behavior of oxide films. In contrast to silicon the stiffness of SiO$_2$ is approximately only a third.

The thermal conductivity is an important parameter which affects power during circuit operation. The stability of H$_2$O under high electric fields is expressed as its dielectric strength which is related to the high resistivity. The bandgap of SiO$_2$ is nearly 8 times wider compared to the bandgap of silicon. The wide bandgap and the high dielectric strength make oxide films very suitable for dielectric isolation.

2.1.2 Structure of SiO $\boldsymbol{_2}$

SiO$_2$ can be described as a three-dimensional network constructed from tetrahedral cells, with four oxygen atoms surrounding a silicon atom [25], as shown in a two-dimensional projection in Fig. 2.1a. The silicon atoms are in the center of each of the tetrahedra. The length of a Si-O bond is 0.162nm and the normal distance between oxygen ions is 0.262nm. The Si-Si bond distance depends on the particular form of SiO$_2$ with about 0.31nm. The six-membered ring structure of SiO$_2$ is shown in Fig. 2.1b. In an ideal network the vertices of the tetrahedra are joined by a common oxygen atom called a bridging oxygen.

Figure 2.1: Structure of fused silica glass a) and structure of SiO$_2$ b).

In the amorphous forms of SiO$_2$ there can be also some non-bridging oxygen atoms. These phases are often named as fused silica. Crystalline forms of SiO$_2$ such as quartz contain only binding oxygen bonds. The various crystalline and amorphous forms of SiO$_2$ arise because of the ability of the bridging oxygen bonds to rotate, allowing the position of one tetrahedron to move with respect to its neighbors. This same rotation allows the material to lose long-range order and hence become amorphous. The rotation and the capability to vary the angle of the Si-O-Si bond from 120$^{\circ }$ to 180$^{\circ }$ with only a little change in energy play an important role in matching amorphous SiO$_2$ with crystalline silicon without breaking bonds [28].